Following years of smoking rates that had hovered around 20 percent, that number finally dropped to 18.1 percent in 2012, statistics from the U.S. Centers for Disease Control and Prevention show.

"This is a milestone. We have seen a steady decline in recent years, and so the stall is no longer occurring," said report co-author Brian King, an epidemiologist in the CDC's Office on Smoking and Health. "But the progress is still not as strong as we would hope."

King said several developments on the "national level helped to galvanize tobacco efforts, and help reduce smoking rates."

"In 2009, the Family Smoking Prevention and Tobacco Control Act was implemented, which gave the FDA [U.S. Food and Drug Administration] the authority to regulate tobacco," he said. "Also, in 2009 the federal tax rate for cigarettes increased from 39 cents a pack to $1.01 a pack. We know that increasing tobacco prices is the single most effective way to reduce smoking."

Another factor might have been the "Tips from Former Smokers" advertising campaign, which the CDC launched in 2012, King said. In the ads, a dozen or so ex-smokers offered harrowing personal tales on the devastating health consequences that can come from years of tobacco use.

One of the most striking ads featured a 53-year-old North Carolina woman named Terrie Hall, who was diagnosed with oral and throat cancer and had to have her voice box removed.

In her first ad, Hall was shown putting on a wig, inserting false teeth and using a scarf to hide a hole in her throat. It was the campaign's most widely viewed ad, and received more than 2.8 million views on YouTube.

"There are still disparities in smoking rates across populations, and there are still 42.1 million American adults who still smoke. We need enhancement of proven strategies, such as increased tobacco pricing, smoke-free laws, hard-hitting mass media in concert with cessation resources to effectively end the tobacco epidemic," he added.

In addition to a drop in the number of adult smokers, the number of cigarettes people are smoking also dropped, King said.

"Among people who smoke the most, those who smoke 30 or more cigarettes a day, there was a decline from 12.6 percent to 7 percent," he said.

And among people who smoke every day, the average number of cigarettes they smoked dropped from 16.7 in 2005 to 14.6 in 2012.

The findings were published in the Jan. 17 issue of the CDC's Morbidity and Mortality Weekly Report.

King noted that smoking rates remain highest among those with the least education (41.9 percent) and those below the poverty level (27.9 percent).

The best way to target those groups is to raise the cost of cigarettes, he said.

King is worried that some smokers are switching from cigarettes to other tobacco products, like little cigars.

"This is an emerging issue as the introduction of novel products has started to proliferate. It is concerning because, although we are seeing declines among certain groups, particularly young adults 18 to 24, the prevalence of these other products is increasing," he said.

Dr. Norman Edelman, senior medical adviser to the American Lung Association, said that "a really important finding to me, which people don't talk about much, is that there was a decrease in the intensity of smoking."

Edelman thinks the fact that people are smoking fewer cigarettes belies the idea that there are some people who are so addicted to cigarettes that they can never quit. "That's important, because this suggests that if there were so-called hard-core nicotine addicts, they wouldn't be smoking less," he said.

Edelman also believes that the most effective way to get people to quit or not to start smoking is to hit them in their pocketbooks.

"The American Lung Association strongly advocates continuing efforts to raise cigarette taxes, because eventually it should discourage people from smoking," he said.

More information

Visit the American Cancer Society (http://www.cancer.org/healthy/stayawayfromtobacco/guidetoquittingsmoking/ ) for more on quitting smoking.

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.